Chapter 3: Anatomy of the Spinal Cord
Nachum Dafny, Ph.D., room of Neurobiology and also Anatomy, The UT clinical School in ~ Houston Reviewed and also revised 07 Oct 2020
Figure 3.1 Schematic dorsal and also lateral check out of the spinal cord and four overcome sections from cervical, thoracic, lumbar and sacral levels, respectively.
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The spinal cord is the most crucial structure in between the body and the brain. The spinal cord extends indigenous the foramen magnum whereby it is consistent with the medulla come the level that the an initial or second lumbar vertebrae. The is a critical link between the mind and the body, and also from the body to the brain. The spinal cord is 40 to 50 centimeter long and 1 cm to 1.5 centimeter in diameter. Two consecutive rows the nerve roots arise on every of that sides. This nerve roots join distally to kind 31 bag of spinal nerves. The spinal cord is a cylindrical structure of nervous tissue composed of white and also gray matter, is uniformly organized and also is divided into four regions: cervical (C), thoracic (T), lumbar (L) and also sacral (S), (Figure 3.1), every of i beg your pardon is made up of several segments. The spinal nerve consists of motor and also sensory nerve yarn to and from all parts of the body. Every spinal cord segment innervates a dermatome (see below and Figure 3.5).
3.2 general Featuressimilar cross-sectional frameworks at every spinal cord level (Figure 3.1). the carries sensory info (sensations) native the body and some indigenous the head come the main nervous system (CNS) via afferent fibers, and also it performs the initial processing of this information. motor neurons in the ventral horn project their axons into the periphery to innervate skeletal and smooth muscles that mediate voluntary and also involuntary reflexes. It contains neurons who descending axons convey autonomic manage for many of the visceral functions. it is of good clinical importance because it is a significant site that traumatic injury and also the locus because that many condition processes.
Although the spinal cord constitutes only around 2% of the central nervous device (CNS), its functions are vital. Knowledge the spinal cord practical anatomy makes it feasible to diagnose the nature and location that cord damage and many cord diseases.
3.3 Segmental and also Longitudinal Organization
The spinal cord is split into four different regions: the cervical, thoracic, lumbar and also sacral areas (Figure 3.1). The different cord regions can be visually distinguished from one another. Two enlargements the the spinal cord deserve to be visualized: The cervical enlargement, i beg your pardon extends between C3 to T1; and also the lumbar enlargements which extends in between L1 to S2 (Figure 3.1).
The cord is segmentally organized. There space 31 segments, characterized by 31 pairs of nerves exiting the cord. This nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and also 1 coccygeal nerve (Figure 3.2). Dorsal and ventral roots enter and also leave the vertebral obelisk respectively with intervertebral foramen in ~ the vertebral segments corresponding to the spinal segment.
Figure 3.2 drawing of the 8, 12, 5, 5 and 1 cervical, thoracic, lumbar, sacral and coccygeal spinal nerves and also their leave from the vertebrate, respectively.
The cord is sheathed in the exact same three meninges together is the brain: the pia, arachnoid and also dura. The dura is the hard outer sheath, the arachnoid lies in ~ it, and the pia closely adheres to the surface of the cord (Figure 3.3). The spinal cord is attached come the dura by a collection of lateral denticulate ligaments create from the pial folds.
Figure 3.3 The 3 spinal cord meninges. The denticulate ligament, the dorsal source ganglion (A), and also an enlarged illustration of the meninges (B).
During the initial 3rd month that embryonic development, the spinal cord expand the entire length that the vertebral canal and both flourish at around the same rate. As breakthrough continues, the body and the vertebral column continue to grow at a much higher rate 보다 the spinal cord proper. This results in displacement the the reduced parts that the spinal cord v relation to the vertebrae column. The outcome of this uneven growth is the the adult spinal cord extends come the level that the an initial or second lumbar vertebrae, and the nerves grow to exit through the exact same intervertebral foramina together they did throughout embryonic development. This development of the nerve roots emerging within the vertebral canal, results in the lumbar, sacral, and also coccygeal roots extending to their suitable vertebral levels (Figure 3.2).
All spinal nerves, other than the first, exit listed below their corresponding vertebrae. In the cervical segments, there room 7 cervical vertebrae and 8 cervical nerves (Figure 3.2). C1-C7 nerves exit above their vertebrae conversely, the C8 nerve exits below the C7 vertebra. It leaves in between the C7 vertebra and the an initial thoracic vertebra. Therefore, each succeeding nerve pipeline the cord listed below the corresponding vertebra. In the thoracic and upper lumbar regions, the difference between the vertebrae and cord level is three segments. Therefore, the source filaments of spinal cord segments have to travel longer ranges to reach the matching intervertebral foramen from which the spinal nerves emerge. The lumbosacral root are recognized as the cauda equina (Figure 3.2).
Each spinal nerve is written of nerve fibers the are related to the an ar of the muscles and skin that creates from one body somite (segment). A spinal segment is defined by dorsal root entering and also ventral root exiting the cord, (i.e., a spinal cord section that provides rise to one spinal nerve is considered as a segment.) (Figure 3.4).
Figure 3.4 (A) illustration of the spinal cord through its spinal roots. (B) illustration of the spinal vertebrate. (C) section of the spinal cord, that meninges and also the dorsal and also ventral roots of 3 segments.
A dermatome is an area that skin gave by peripheral nerve fibers originating from a single dorsal source ganglion. If a nerve is cut, one loser sensation from the dermatome. Since each segment that the cord innervates a different an ar of the body, dermatomes deserve to be specifically mapped ~ above the body surface, and loss of sensation in a dermatome can indicate the exact level of spinal cord damage in clinical evaluate of injury (Figure 3.5). It is essential to take into consideration that there is some overlap in between neighboring dermatomes. Due to the fact that sensory info from the human body is relayed to the CNS through the dorsal roots, the axons originating from dorsal root ganglion cells room classified as primary sensory afferents, and the dorsal root"s neurons space the first order (1°) sensory neuron. Most axons in the ventral roots arise from engine neurons in the ventral horn that the spinal cord and also innervate skeleton muscle. Others arise native the lateral horn and synapse top top autonomic ganglia that innervate visceral organs. The ventral root axons sign up with with the peripheral processes of the dorsal source ganglion cell to type mixed afferent and also efferent spinal nerves, which unify to type peripheral nerves. Understanding of the segmental innervation of the cutaneous area and the muscle is necessary to diagnose the site of an injury.
Figure 3.5 Innervation developing from single dorsal root ganglion supplied details skin area (a dermatome). The numbers describe the spinal segments by which each nerve is named C = cervical; T = thoracic; together = lumbar; S = sacral spinal cord segment (dermatome).
3.4 internal Structure that the Spinal Cord
A transverse section of the adult spinal cord reflects white matter in the periphery, gray issue inside, and also a tiny central canal filled with CSF at its center. Neighboring the canal is a solitary layer of cells, the ependymal layer. Bordering the ependymal layer is the gray matter – a an ar containing cell bodies – shaped favor the letter “H” or a “butterfly”. The 2 “wings” that the butterfly room connected across the midline by the dorsal gray commissure and also below the white commissure (Figure 3.6). The shape and size of the gray matter varies according to spinal cord level. In ~ the reduced levels, the ratio between gray matter and white issue is greater than in higher levels, mainly due to the fact that lower level contain much less ascending and descending nerve fibers. (Figure 3.1 and also Figure 3.6).
The gray matter mainly includes the cabinet bodies of neurons and glia and is separated into four main columns: dorsal horn, intermediary column, lateral horn and also ventral horn column. (Figure 3.6).
The dorsal horn is uncovered at every spinal cord levels and also is comprised of sensory nuclei that obtain and process incoming somatosensory information. Native there, ascending projections emerge to transmit the sensory information to the midbrain and diencephalon. The intermediary column and also the lateral horn make up autonomic neurons innervating visceral and pelvic organs. The ventral horn comprises motor neurons that innervate skeletal muscle.
At all the levels of the spinal cord, nerve cell in the gray substance are multipolar, varying lot in your morphology. Many of them space Golgi kind I and also Golgi type II nerve cells. The axons that Golgi kind I room long and also pass the end of the gray matter into the ventral spinal root or the fiber tracts of the white matter. The axons and dendrites that the Golgi kind II cell are mainly confined to the bordering neurons in the gray matter.
A much more recent category of neurons within the gray issue is based upon function. These cells are situated at every levels that the spinal cord and are grouped right into three main categories: source cells, shaft or street cells and also propriospinal cells.
The root cells are located in the ventral and also lateral gray horns and also vary considerably in size. The most prominent functions of the root cells are big multipolar elements exceeding 25 µm of your somata. The root cells contribute their axons come the ventral roots of the spinal nerves and also are grouped right into two major divisions: 1) somatic efferent root neurons, i m sorry innervate the skeleton musculature; and also 2) the visceral efferent root neurons, likewise called preganglionic autonomic axons, i beg your pardon send their axons to miscellaneous autonomic ganglia.
The column or tract cells and their processes are located greatly in the dorsal gray horn and also are confined completely within the CNS. The axons the the pillar cells type longitudinal ascending tracts that ascend in the white columns and also terminate top top neurons situated rostrally in the mind stem, cerebellum or diencephalon. Some obelisk cells send your axons up and also down the cord to end in gray matter close to their origin and are recognized as intersegmental association obelisk cells. Other tower cell axons terminate in ~ the segment in which castle originate and also are called intrasegmental association column cells. Still other pillar cells send their axons across the midline to end in gray matter close to your origin and are dubbed commissure association shaft cells.
The propriospinal cells space spinal interneurons whose axons do not leaving the spinal cord proper. Propriospinal cells account for around 90% that spinal neurons. Few of these fibers likewise are found roughly the margin that the gray matter of the cord and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract.
3.5 Spinal Cord Nuclei and Laminae
Spinal neurons room organized into nuclei and also laminae.
The influential nuclear teams of cell columns in ~ the spinal cord native dorsal come ventral room the marginal zone, substantia gelatinosa, nucleus proprius, dorsal cell core of Clarke, intermediolateral nucleus and also the lower motor neuron nuclei.
Marginal zone nucleus or posterior marginalis, is found at every spinal cord levels as a thin layer the column/tract cell (column cells) the caps the tip of the dorsal horn. The axons of its neurons add to the lateral spinothalamic tract which relays pain and temperature info to the diencephalon (Figure 3.7).
Substantia gelatinosa is found at all levels of the spinal cord. Located in the dorsal cap-like part of the head that the dorsal horn, that relays pain, temperature and mechanical (light touch) information and also consists greatly of column cells (intersegmental obelisk cells). These column cells synapse in cell at Rexed layers IV come VII, who axons contribute to the ventral (anterior) and also lateral spinal thalamic tracts. The homologous substantia gelatinosa in the medulla is the spinal trigeminal nucleus.
Nucleus proprius is located below the substantia gelatinosa in the head and also neck that the dorsal horn. This cell group, sometimes dubbed the chef sensory nucleus, is connected with mechanical and also temperature sensations. That is a poorly characterized cell tower which extends with all segment of the spinal cord and its neurons contribute to ventral and also lateral spinal thalamic tracts, as well as to spinal cerebellar tracts. The axons originating in cell nucleus proprius task to the thalamus via the spinothalamic tract and also to the cerebellum via the ventral spinocerebellar tract (VSCT).
Dorsal cell nucleus of Clarke is a cell column situated in the mid-portion the the base form of the dorsal horn. The axons from these cells pass uncrossed come the lateral funiculus and form the dorsal (posterior) spinocerebellar tract (DSCT), i m sorry subserve unconscious proprioception indigenous muscle spindles and Golgi tendon organs come the cerebellum, and also some of them innervate spinal interneurons. The dorsal cell nucleus of Clarke is uncovered only in segment C8 to L3 of the spinal cord and is most prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accessory cuneate nucleus, i beg your pardon is the beginning of the cuneocerebellar street (CCT).
Intermediolateral nucleus is situated in the intermediary zone in between the dorsal and the ventral horns in the spinal cord levels. Prolonging from C8 come L3, that receives viscerosensory information and also contains preganglionic sympathetic neurons, which type the lateral horn. A huge proportion that its cells space root cells which send axons into the ventral spinal root via the white rami to reach the sympathetic tract together preganglionic fibers. Similarly, cabinet columns in the intermediolateral nucleus located at the S2 to S4 levels has preganglionic parasympathetic neurons (Figure 3.7).
Lower motor neuron nuclei are located in the ventral horn the the spinal cord. Lock contain mainly motor nuclei consist of of α, β and also γ motor neurons and are discovered at all levels the the spinal cord--they room root cells. The a engine neurons are the final typical pathway that the engine system, and they innervate the visceral and also skeletal muscles.
3.7 Rexed Laminae
The distribution of cells and also fibers within the gray issue of the spinal cord exhibits a sample of lamination. The cellular pattern of every lamina is created of miscellaneous sizes or forms of neurons (cytoarchitecture) which led Rexed to suggest a brand-new classification based on 10 layers (laminae). This group is useful because it is related an ext accurately to duty than the previous group scheme which was based on significant nuclear groups (Figure 3.7).
Laminae ns to IV, in general, are involved with exteroceptive sensation and also comprise the dorsal horn, vice versa, laminae V and VI are concerned primarily v proprioceptive sensations. Lamina VII is indistinguishable to the intermediate zone and also acts as a relay in between muscle spindle to midbrain and also cerebellum, and laminae VIII-IX make up the ventral horn and also contain mainly motor neurons. The axons of this neurons innervate greatly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia.
Rexed lamina i – is composed of a slim layer of cells the cap the guideline of the dorsal horn with little dendrites and also a complicated array of nonmyelinated axons. Cell in lamina i respond greatly to noxious and also thermal stimuli. Lamina i cell axons sign up with the contralateral spinothalamic tract; this layer coincides to cell core posteromarginalis.
Rexed lamina II – created of tightly packed interneurons. This layer synchronizes to the substantia gelatinosa and responds to noxious stimuli while rather respond to non-noxious stimuli. The majority of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells and also descending dorsolateral fasciculus (DLF) fibers. Castle send axons come Rexed laminae III and also IV (fasciculus proprius). High concentrations of substance P and also opiate receptors have actually been identified in Rexed lamina II. The lamina is believed to be important for the modulation the sensory input, v the effect of determining which sample of incoming information will create sensations that will certainly be understood by the brain as being painful.
Rexed lamina III – created of variable cabinet size, axons of this neurons bifurcate number of times and form a thick plexus. Cells in this layer receive axodendritic synapse from Aβ fibers entering dorsal root fibers. It contains dendrites of cell from laminae IV, V and also VI. Most of the neurons in lamina III function as propriospinal/interneuron cells.
Rexed lamina IV – The thickest that the an initial four laminae. Cells in this layer obtain Aß axons which lug predominantly non-noxious information. In addition, dendrites of neurons in lamina IV radiate to lamina II, and respond come stimuli such together light touch. The ill-defined nucleus proprius is located in the head the this layer. Some of the cells task to the thalamus via the contralateral and also ipsilateral spinothalamic tract.
Rexed lamina V – created neurons with their dendrites in lamina II. The neurons in this lamina get monosynaptic info from Aß, advertisement and C axons which likewise carry nociceptive details from visceral organs. This lamina consists a large zone extending across the neck of the dorsal horn and is separated into medial and lateral parts. Many of the Rexed lamina V cells job to the brain stem and the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, diminish corticospinal and rubrospinal fibers synapse top top its cells.
Rexed lamina by means of – Is a vast layer which is best emerged in the cervical and also lumbar enlargements. Lamina by means of divides likewise into medial and also lateral parts. Team Ia afferent axons from muscle spindles end in the medial component at the C8 come L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the little neurons are interneurons participating in spinal reflexes, while descending brainstem pathways job to the lateral zone of Rexed class VI.
Rexed lamina VII – This lamina occupies a large heterogeneous region. This an ar is also known together the zona intermedia (or intermediolateral nucleus). That shape and boundaries vary along the size of the cord. Lamina VII neurons receive details from Rexed lamina II come VI as well as visceral afferent fibers, and also they offer as an intermediary relay in transmission of visceral motor neurons impulses. The dorsal cell nucleus of Clarke develops a significant round oval cell pillar from C8 to L3. The large cells give rise to uncrossed nerve yarn of the dorsal spinocerebellar street (DSCT). Cells in laminae V to VII, which carry out not kind a discrete nucleus, provide rise to uncrossed yarn that kind the ventral spinocerebellar street (VSCT). Cells in the lateral horn the the cord in segments T1 and also L3 provide rise to preganglionic sympathetic yarn to innervate postganglionic cells located in the sympathetic ganglia outside the cord. Lateral horn neurons at segment S2 to S4 provide rise come preganglionic neurons the the sacral parasympathetic yarn to innervate postganglionic cells situated in peripheral ganglia.
Rexed lamina VIII – contains an area in ~ the base of the ventral horn, however its form differs at miscellaneous cord levels. In the cord enlargements, the lamina occupies just the medial part of the ventral horn, whereby descending vestibulospinal and reticulospinal yarn terminate. The neurons of lamina VIII modulate motor activity, most probably via g motor neurons i m sorry innervate the intrafusal muscle fibers.
Rexed lamina IX – composed of several unique groups of huge a motor neurons and tiny γ and β motor neurons installed within this layer. The size and also shape different at various cord levels. In the cord enlargements the number of α motor neurons increase and also they type numerous groups. The α engine neurons are large and multipolar cells and also give increase to ventral root fibers to it is provided extrafusal skeleton muscle fibers, if the small γ engine neurons offer rise come the intrafusal muscle fibers. The α engine neurons room somatotopically organized.
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Rexed lamina X – Neurons in Rexed lamina X surround the central canal and also occupy the commissural lateral area that the gray commissure, which likewise contains decussating axons.
In summary, laminae I-IV are involved with exteroceptive sensations, conversely, laminae V and VI are came to primarily with proprioceptive sensation and act together a relay between the periphery to the midbrain and also the cerebellum. Laminae VIII and IX kind the last motor pathway to initiate and also modulate motor task via α, β and γ engine neurons, i m sorry innervate striated muscle. Every visceral motor neurons are situated in lamina VII and innervate neurons in autonomic ganglia.
3.8 White Matter
neighboring the gray matter is white matter containing myelinated and also unmyelinated nerve fibers. This fibers conduct info up (ascending) or down (descending) the cord. The white matter is divided into the dorsal (or posterior) shaft (or funiculus), lateral column and ventral (or anterior) pillar (Figure 3.8). The anterior white commissure lives in the center of the spinal cord, and it has crossing nerve fibers the belong come the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three basic nerve fiber species can be distinguished in the spinal cord white matter: 1) long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in assorted brainstem nuclei, cerebellum and also dorsal thalamus, 2) long descending nerve yarn originating native the cerebral cortex and also various brainstem nuclei come synapse within the different Rexed layers in the spinal cord gray matter, and also 3) shorter nerve fibers interconnecting assorted spinal cord levels such together the yarn responsible for the coordination that flexor reflexes. Ascending tracts are uncovered in all columns conversely, descending tracts are uncovered only in the lateral and the anterior columns.